events: Only export internal events to ics

mpicms/events/models.py

@@ -119,7 +119,7 @@ def clean(self):  # Prevent more than one event index
     @property
     def ics(self):
         c = Calendar()
-        for event in self.events:
+        for event in self.events.filter(external=False):
             e = ICSEvent(
                 name=event.title,
                 begin=event.start,